What is the result you want? Is it a library? executable? Something else? Whatever it is, you put that on the left of the colon, a list of the required prerequisite "things" (object files, libraries etc) on the right, and how to put them together to make your result on the next line, tab indented. For instance, lines 25 and 26 correctly say: To make "clean" (on the left of the colon), there are no prerequisites (on the right of the colon), and you do it by using the rm -f command on $(OBJS) and $(TARGET) (on the tab indented line below the target and prerequisites line
I don't really understandline 15: Why is TARGET a list of c files?
line 17: My version of make (GNU Make 3.81) creates OBJS = build_grid.c.o cheb_2D_tring.c.o etc. Very probably not what you wanted.
line 19: Says how to build $(TARGET) which is a list of c files. Almost certainly not what you wanted, particularly considering line 20
so... line 13 is kind of moot until you fix the other issues.
Suggest you do a brief tutorial on makefiles. Here's one: http://makepp.sourceforge.net/1.19/makepp_tutorial.html
And of course, here's the definitive GNU make manual